Independent regulation of mouse VL30 retrotransposon expression in response to serum and oncogenic cell transformation.

The nucleotide sequence of the long terminal repeats (LTRs) of retrovirus-transmissible mouse VL30 cDNA clones, NVL-1 and NVL-2 were determined and compared with that of the prototype NVL-3. Both shared the typical U3 R U5 structure together with unusual features of redundancy in the tRNAgly primer binding site and adjacent inverted repeat. NVL-1 and NVL-2 LTRs were almost identical and differed from the NVL-3 LTR in the U3 domain harbouring transcriptional regulatory determinants. S1 nuclease analysis of cellular and virus-encapsidated RNA suggested that NVL-1/2 and NVL-3 elements retrotranspose with comparable efficiency but that in contrast to transformation-regulated VL30 expression which affects all types of NVL element, only NVL-1/2 elements were found to be serum responsive. Both modes of VL30 regulation were found to be coupled through protein kinase C-independent pathways. Expression of N-ras transactivated U3 enhancer determinants in all classes of LTR. However the same region of NVL-1/2 LTR did not confer serum responsiveness implying that cis regulatory determinants of VL30 elements mediating growth factor responsiveness are at least in part dissociable from those responsible for cell transformation-regulated expression.